1. Field of the Invention
The invention relates to providing values of components of total carbon dioxide excreted by a subject.
2. Description of the Related Art
Carbon dioxide (CO2) excretion monitoring is done, for example, to assess the adequacy of mechanical ventilation and the rate of metabolism in critical care subjects. Carbon dioxide excretion measurements are generally made by integrating respiratory flow and carbon dioxide signals over time. Carbon dioxide is a natural by-product of metabolism and is excreted with every breath. The human body stores a large amount of carbon dioxide in tissues as dissolved gas, bound to bicarbonate, and/or in other forms. As such, a measured carbon dioxide excretion rate reflects the sum of the metabolically produced carbon dioxide and the rate of carbon dioxide transfer to or from the carbon dioxide stores in the body. Carbon dioxide excretion is often expressed as the volume of excreted carbon dioxide per minute, and may be represented as VCO2. Typically, a human excretes approximately 200 mL of carbon dioxide per minute.
The rate of metabolic carbon dioxide production is proportional to whole-body metabolic activity. If the exact substrate utilization is known, respiratory quotient (RQ) can be estimated and energy expenditure can be calculated directly from the metabolic carbon dioxide production rate. Metabolic carbon dioxide production increases with increasing caloric expenditure caused by accelerated physical activity, increased respiratory effort, sepsis, malignant hyperthermia etc. Metabolic carbon dioxide production decreases in response to a decrease in metabolic activity caused by a decrease in subject work or organ failure.
Transfer of carbon dioxide to, or from, the tissue stores follows a change in effective ventilation. An increase in ventilation causes an increase in carbon dioxide excretion. In this situation, carbon dioxide excretion represents the sum of carbon dioxide released from the stores plus metabolically produced carbon dioxide. A decrease in effective ventilation causes a fall in rate of carbon dioxide excretion. When the body is unable to excrete all of the metabolically produced carbon dioxide, the measured carbon dioxide excretion is the metabolically produced carbon dioxide less the amount is transferred to the stores. The rate at which carbon dioxide is added to, or released from, the stores indicates the rate of change in the partial pressure of arterial carbon dioxide (PaCO2). The transfer of carbon dioxide to and from the stores following changes in ventilation or metabolic rate continues for an extended period depending on subject size, cardiac output etc.
When a subject is over-ventilated, measured carbon dioxide excretion is in excess of metabolic carbon dioxide production and carbon dioxide is being “blown off,” or removed, from the carbon dioxide stored in the tissues, which leads to a reduction in arterial carbon dioxide. When a subject is under-ventilated, carbon dioxide excretion is inadequate to clear all of the metabolically created carbon dioxide, so carbon dioxide accumulates in the tissues, causing arterial carbon dioxide to rise.
When ventilation and metabolic rate have been stable for a long time (1-2 hours depending on the subject and conditions) then the measured carbon dioxide excretion is approximately equal to the rate of metabolically produced carbon dioxide. By definition, this is the period of steady state when carbon dioxide is not being transferred to or from the tissue stores and therefore the measured carbon dioxide excretion solely reflects the carbon dioxide from metabolism. In many critical care situations, however, ventilation is unstable such that the measured carbon dioxide excretion is the sum of metabolically produced carbon dioxide and transfers to or from carbon dioxide stores.
The utility of measured carbon dioxide excretion as a clinically monitored parameter is limited because the measured rate of carbon dioxide excretion is the sum of metabolic carbon dioxide production and transfer of carbon dioxide to or from tissue stores. In the presently available volumetric capnometry products, there is no way in a clinical setting to separate the rate of metabolic carbon dioxide production from the rate of carbon dioxide transfer to and from the tissue carbon dioxide stores.